fix(pdk): response.set_header support header argument with table array of string

[oowl]

Summary

  transform_headers(conf, kong.response.get_headers())
  -> ngx.resp.get_headers(max_headers)

In Openresty, if we set the response header in the access(or rewrite) phase, it will add headers to r->headers_out, and continue to other logic, and then real upstream processing and return headers to Openresty C logic, Openresty will add these response headers (return from real upstream) to r->headers_out, the default logic can not overwrite existing header which set by ngx.header["bala"] = "ball", So if we set the ngx.header["X-ratelimit-limit"] = 10 header in access phase, but real upstream also return “X-ratelimit-limit" header, The Openresty will store these two headers in r->headers_out, And iterate r->headers_out all response header to downstream, But for ngx.resp.get_headers APi, it will return table(not string) which contains two X-ratelimit-limit headers value.

https://github.com/openresty/lua-resty-core/blob/50f73790b6bb1d83f52cb3b4ec7e4ab7f649ab32/lib/resty/core/response.lua#L231

https://github.com/openresty/lua-nginx-module/blob/master/src/ngx_http_lua_headers.c#L1043

https://github.com/openresty/lua-nginx-module/blob/master/src/ngx_http_lua_headers_out.c#L552

https://github.com/openresty/lua-nginx-module/blob/master/src/ngx_http_lua_headers_out.c#L140

So for Kong, if the response header has two same headers (one header set by ngx.req.header), it will merge two headers to a table returned by ngx.resp.get_headers()

In response-header-transformer, it will cause a type error, this fix tries to let response.set_header support the header argument with a table array of string
https://github.com/Kong/kong/blob/master/kong/plugins/response-transformer/header_transformer.lua#L99

https://github.com/Kong/kong/pull/12164/files#diff-c9c2f298c974df55f211340f437466cf70d690dde66ae40dfaf7e1a3c0e3c859R103

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

FTI-5585

[windmgc]

Good catch!

[ms2008]

The description here also needs to be updated https://docs.konghq.com/gateway/latest/plugin-development/pdk/kong.response/#kongresponseset_headername-value

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-12164-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-12164-to-master-to-upstream
git checkout -b cherry-pick-12164-to-master-to-upstream
ancref=$(git merge-base 6fe681348bb0a58efbbbc5f2a6ef57828ed61667 a47bc6f43cc9162e63ca498b2d476826840dc2af)
git cherry-pick -x $ancref..a47bc6f43cc9162e63ca498b2d476826840dc2af